The present invention relates to kraft chemical recovery boilers and more particularly to the air intake ports of such boilers. In even greater particularity, the present invention relates to the maintenance of such air ports to facilitate the improved efficiency of such boilers. In still greater particularity the present invention relates to apparatus for damping selected ones of the primary, secondary, or tertiary air ports of the boiler such that automated rodding apparatus may be employed to clear the port on a scheduled or as needed basis.
Many black liquor recovery boilers experience problems with unscheduled shutdowns due to excessive build-up on the heat transfer surfaces of carryover or fume particles. The carryover particles are black liquor droplets entrained by the combustion gases carried into the upper furnace. The fume particles are formed from the condensation of sodium and related compounds, which in turn are emitted in gaseous form from the burning liquid droplets or smelt bed. Generally, plugging in the superheater and boiler bank is normally formed by deposits of carryover rather than fume. Thus, the quantity of carryover present in the upper furnace has a significant impact on the pluggage rate.
The Particulator Carryover Monitoring System manufactured by Quadtek, Inc. uses an infrared camera which is installed in the upper furnace facing the superheater tubes in a region in which the air flow patterns are constant, avoiding recirculation zones. The video signal from the upper furnace camera is digitized under the control of a computer which counts particles that cross a horizontal line in the middle of the monitor screen. The Particulator's image processor superimposes two (2) arrows on the monitor which defines the line that the computer scans. The particles that pass between the arrows are counted. The accumulated particle counts are displayed in counts per second (CPS) and counts per minute (CPM) .
A typical recovery boiler operating at a constant firing rate shows significant carryover fluctuation on an hourly basis. The hourly increases are believed to be associated with manual primary air port rodding. The average primary air pressure without continuous rodding was 3.4" H.sub.2 O with an average deviation of 0.5" H.sub.2 O. During continuous manual rodding, primary air pressure was reduced to an average of 2.53" H.sub.2 O with an average standard deviation of 0.145" H.sub.2 O.
The primary air accounts for approximately sixty (60) percent of the combustion air. As the ports experience smelt build-up, the air flow rate changes by as much as ten (10) percent. When the smelted ports are manually rodded on two (2) hour intervals the boiler experiences a surge of primary air. This surge can cause particulate from the char bed to be entrained into the flue gas because of the sweeping action of the primary air over the char bed and leads to increased carryover and char bed shape changes. The surges in primary air were eliminated during continuous manual port rodding resulting in a substantial reduction in carryover and a stabilization of the primary air pressure. The frequency of the manual rodding is dependent upon the physical and operational characteristics of the boiler.
Thus, although the advantages of automatic rodding are known, the practice of automatic rodding is not universal and has been impeded by the necessity to provide dampers in the air flow path through an associated wind box to the boiler ports. The known dampers are as shown in FIG. 7 of the drawings submitted herewith and are such that the automatic rodding devices are incompatible with automatic damping. That is to say in known damped systems, the rodding must be performed in a manual or at best semi automated mode because, as seen in FIG. 7 the operator, manually or with an actuator A, must move the damper D using lever L from intermediate the boiler port P and the access port R in the wind box W to enable the rodding device to gain access to the port P for cleaning. Alternatively the damper must be positioned distal the port and thus loses some of its efficiency by virtue of its remote location.